Having seen all the good ideas here and now having the peek preview of Ancient Magic, getting an impression on what it is and isnt, here are what I would like the most:
The Order of Hermes.
An overview of the Order, its movers and shakers, and the Tribunals. Just a short tour of Mythic Europe and how the tribunals look and work. More on the background of the Order. Chapters dedicated the Cult of Mercury, the Germeanic traditions and most of all the Diedne. And give us loads of plothooks. I really like the great use of plothooks in 5th material and especially the Fallen Fane included many hooks that could be introduced here or expanded upon. I see this book as a home for plothooks involving turning events for the Order as a whole. In depths highlights of Hermetic history. The Sundering, and surely the Schism War. It really seems from this thread that there is plenty of interest in the Schism War. A pity that my Schism War-themed saga will probably be over before we see anything on it, but I would still be like a kid at the christmas tree for getting my paws on such a book.
The Wizards Grimoire
I realise that much of the material in the former Grimoires have been adopted to many of the existing 5th books or become obsolete with the new rules. But there is still a lot of the material that might be updated and new stuff to be added. And more plot hooks here aswell.
The book of Illuminations
I am very visual. I love going to places and museums of medieval history. And I also have quite a collection of props and visual aides I never know if I will ever get to use in a session, but just in case.. I can't help buying them. And I often spend time working with graphics and layout to produce handouts, again because I love to have some kinds of off-set for your imagination. That is why I would love a book ripe with great artwork showing the daily life of magi, documents, correspondances between sodales, illustrations from labnotes and in-setting illuminations. The better a medieval feel the merrier! This book could also include small stories in themselves - not RPG stories, but short stories which might very well inspire aswell, but stories foremost.
"a Fallen Fane-sequel"
Well actually not a sequel, but just another interesting live scenario. Live scenarios are a bit harder to make yourself than the regular stories we are doing on a regular basis. Fallen Fane was a very interesting and well done scenario. Half of my troupe actually backed our bags and went across the country for a few days to play Fallen Fane with some other Ars-troupes. Added to that I think live scenarios are a great way to play certain aspects of life in the Order - Tribunals being the most obvious thing - and it is also a great avenue to introduce new players to the game (as they only have to learn the setting and not the rule mechanics).
On top of this I would also give support to a short introductionary booklet including introduction to the setting, some of the rules, and an introduction story. This need not be terribly advanced or expensive printing, actually the better if it wasn't, but something we might advocate or distribute to local friends, networks, clubs, FLGS and conventions.